This invention relates to an apparatus and method for extracting data values from a signal encoded with AES3 data.
The Audio Engineering Society and the European Broadcasting Union have established a standard which describes a signal format for distributing two channels of audio material in a digital data stream. The data stream, which is frequently referred to as the AES3 data stream, is composed of frames, and each frame of the AES3 data stream is composed of two channel subframes, which are referred to as subframe A and subframe B, in sequence. The data stream utilizes a biphase mark coded signal. Each source data bit is represented by a two-cell doublet. Each doublet that represents a source data bit begins, and therefore also ends, with a transition. A source data bit 0 generates a doublet with no further transition before the end of the doublet, whereas a source data bit 1 generates a transition between the cells of the doublet. Thus, a source data bit 1 will be represented by the doublet 01 or 10 and a source data bit 0 will be represented by the doublet 00 or 11. Each subframe has 32 doublets and contains data representing one audio sample. In accordance with the AES3 standard, the first four doublets of each subframe constitute a preamble containing at least one occurrence of the three cell sequence 000 or 111. This brief departure from the usual biphase mark coding rules is known as a code violation.
It will be appreciated from the foregoing that a signal encoded with an AES3 data stream is composed of pulses, and each pulse is one, two or three bit cells in duration. In the following description, a pulse that is one bit cell in duration is referred to as a half pulse, a pulse that is two bit cells in duration is referred to as a whole pulse, and a pulse that is three bit cells in duration is referred to as a wide pulse.
The AES3 standard does not prescribe a sample rate and accordingly in a given facility there may be items of equipment operating at different sample rates, such as 44.1 kHz, 48 kHz and 96 kHz. Hitherto, in order to extract the data from an AES3 data stream, it has been necessary to recover the clock information from the data stream and use the recovered clock to sample the signal. This approach is not optimum because a circuit that is designed for extracting data from a data stream at a sample rate of 44.1 kHz might not be able to extract data reliably from a data stream at a sample rate of 96 kHz.
In accordance with a first aspect of the invention there is provided apparatus for determining nominal pulse duration values in a signal encoded with an AES3 data stream, comprising a first circuit for measuring duration of each pulse of the signal and providing a sequence of duration values, and a second circuit for detecting a maximum duration value, corresponding to duration of three bit cells, and providing first and second duration values corresponding to one bit cell and two bit cells respectively.
In accordance with a second aspect of the invention there is provided apparatus for extracting sample data values from a signal encoded with an AES3 data stream, comprising a first circuit for measuring duration of each pulse of the signal and providing a sequence of duration values, and a second circuit for receiving at least one nominal pulse duration value, corresponding to nominal duration of a predetermined number of bit cells, and comparing the measured duration value with at least one nominal pulse duration value to determine the duration of the pulse relative to the nominal pulse duration value.
In accordance with a third aspect of the invention there is provided a method for determining nominal pulse duration values in a signal encoded with an AES3 data stream, comprising measuring duration of each pulse of the signal and providing a sequence of duration values, and detecting a maximum duration value, corresponding to duration of three bit cells, and providing first and second duration values corresponding to one bit cell and two bit cells respectively.
In accordance with a fourth aspect of the invention there is provided a method for extracting sample data values from a signal encoded with an AES3 data stream, comprising measuring duration of each pulse of the signal and providing a sequence of duration values, receiving at least one nominal pulse duration value, corresponding to nominal duration of a predetermined number of bit cells, and comparing the measured duration value with at least one nominal pulse duration value to determine the duration of the pulse relative to the nominal pulse duration value.